Geologic study of the subsurface structure of the Earth continues to be an important field of endeavor. The continued search for producing reservoirs of hydrocarbons, such as oil and gas, is a particularly important motivation for conducting such studies and obtaining information concerning the Earth's subsurface structure.
One potential source of this information is the measurement of acoustic velocity in outcrops or rock samples. Changes in acoustic velocity can indicate regions of similar or dissimilar material and can also provide estimates of homogeneity and/or anistropy. Such information helps to predict the occurrence of hydrocarbon reservoir traps and provides information about the porosity of materials which is important for hydrocarbon production.
Acoustic velocity can also be helpful in correlating seismic data to well cores and can aid in fault detection. All of such information can lead to additional discoveries of hydrocarbons and to the increase in production from reservoirs. Thus, acoustic velocity information is an important piece of information in the exploration and production of hydrocarbons.
In addition, measurement of acoustic properties has proved useful in non-destructive materials testing applications such as fracture and flow detection, grain and lamination orientation determination, bond integrity evaluation, and material identification.
There are currently numerous hand-held or portable ultrasonic devices which are commercially available. Almost all of these existing devices are used for distance or thickness measurements and not velocity measurements. Ultrasonic thickness gauges are often used on pipes and other equipment and marine sonic systems are commonly used as depth sounders or fish finders. Some ultrasonic devices have been used to measure the flow rate or movement velocity of a liquid. In addition, sonic logs of boreholes have been useful in exploration and production of hydrocarbons.
More recently, acoustic velocity measurement of rock samples has been accomplished using hand-held apparatus. Such hand-held apparatus are known in the art and include a single source and single receiver transducer a fixed distance apart. The source transducer imparts acoustic energy into the sample. The resulting acoustic wave travels through the sample and, given the fixed distance between source and receiver, its velocity is calculated using the difference in time between transmission and arrival at the receiver. Unfortunately, the current systems are susceptible to poor coupling between the transmitting transducer and the rock sample and are also susceptible to errors caused by differences between the transmitted acoustic energy wave form and received wave form resulting from inherent properties of the rock sample. Much effort has been applied to developing an acoustic velocity measuring system which eliminates such potential errors.